The performance of axial flow compressors and/or axial flow turbines is limited by the clearance between the blade tips and the casing.
Conventionally an abradable lining is provided on the inner surface of the casing or onto the inner surface of a plurality of circumferentially arranged shroud segments. The abradable lining is worn away during the initial engine running to produce a minimum clearance between the blade tips and the casing or shroud segments. However, at maximum power there is a maximum amount of wear of the abradable lining and this leads to an increased clearance between the blade tips and the casing or shroud segments at other operating conditions and hence there is a loss of efficiency at cruise conditions.
Considerable effort has been made to minimise this effect, but it can only be minimised and not removed.
Additionally there is a requirement for a seal around the whole of the circumference of the casing, particularly for a compressor, which is not possible with shroud segments due to the leakage flow between the circumferentially adjacent shrouds. Also there is a requirement for very stiff casings to maintain their roundness to maintain a small clearance around the full circumference of the casing.